Biosynthesis of the Diarrhetic Shellfish Poisoning Toxin by the Marine Dinoflagellate Prorocentrum lima

Among the human illnesses that are associated with the consumption of contaminated seafood are ciguatera fish poisoning, neurotoxic shellfish poisoning and diarrhetic shellfish poisoning (DSP). Symptoms are traced to the consumption of toxins that are concentrated in the food chain, but have also been isolated from laboratory cultures of dinoflagellates, a group of marine phytoplankton. Pure dinoflagellate cultures, free of bacteria, have been nearly impossible to obtain. This raises the question of the ultimate origins of dinoflagellate-associated toxins. These toxins, ciguatoxin, brevetoxin and okadaic acid belong to a larger group of secondary metabolites called polyketides. Polyketides share a common biosynthetic pathway, hence the enzymes that produce polyketides, and in turn the DNA that codes for those enzymes, are highly homologous. PCR primers, complementary to conserved motifs have been used to amplify polyketide biosynthetic genes from cultures of the DSP toxin producing dinoflagellate Prorocentrum lima. These amplified products can now be used as probes for two purposes: 1) hybridization of fluorescently labeled DNA probes to identify the polyketide producing microorganism from a mixed assemblage, and 2) probing genomic libraries with DNA probes to locate the entire biosynthetic pathway. A thorough understanding of the biosynthesis of these toxins; including the identification of the producing organism; is the first step toward strategies for detection; mitigation or control of toxin biosynthesis in the environment.